The invention relates to a method of controlling or stabilizing the attitude of a vehicle in space.
In general, the attitude of a space vehicle, such as a satellite, is controlled on the basis of a frame of reference having three axes: a roll axis, a pitch axis, and a yaw axis, and the satellite can be turned about each of these axes, with the magnitude of the turn implemented by the space vehicle about each of these three axes being referred to as the roll, pitch, and yaw angles, respectively.
Document U.S. Pat. No. 5,205,518 discloses a system for controlling the attitude of a space vehicle, and which operates without using gyros. According to that document, the satellite presents substantially zero angular momentum relative to the earth, towards which the yaw axis of the satellite is directed, whereas its roll axis is parallel to the path it is following. In general, the system measures the roll and pitch angles of the satellite by using a detector or a sensor referenced on the earth, and it estimates the yaw angle without measuring it directly. This estimate is based on calculating the values of the roll and yaw torques which are determined from the speeds of rotation of associated reaction wheels by applying equations that are representative of dynamic models for making such an estimate.
Thus, in the prior art, the attitude of the space vehicle is determined about two reference axes on the basis of angle values measured by a sensor pointing towards a heavenly body, whereas the attitude of the vehicle about the third axis is not measured, but is estimated on the basis of predetermined models.
An object of the invention is likewise to design a method of controlling the attitude of a space vehicle without using gyros, and which implements a solution different from the solution used in the above-mentioned prior art, the present solution being based on the observation that the reaction wheels reflect conservation of the total angular momentum of the space vehicle, this making it possible to determine the attitude of the space vehicle about the third reference axis on the basis of a measurement which is obtained by computation based directly on the speeds of rotation of the reaction wheels, and without making use of mathematical models.
To this end, the invention provides a method of controlling the attitude of a vehicle in space relative to three reference axes, about each of which the space vehicle can be caused to turn, wherein, knowing that speeds of rotation of the three reaction wheels reflect the conservation of total angular momentum of the space vehicle, the method consists:
in measuring the angles of rotation of the space vehicle about two of the three reference axes by means of a two-axis detector pointing towards a reference heavenly body, e.g. the sun, in controlling the speeds of rotation of the reaction wheels to set the attitude of the space vehicle initially relative to the desired angles of rotation about these first two reference axes, and then
in measuring the speeds of rotation of the three reaction wheels to calculate the components of the vector representative of the non-zero angular momentum of the space vehicle in a transverse plane extending transversally to a line passing through the space vehicle and the reference heavenly body, said transverse plane containing the first two reference axes; and
in calculating the angle of rotation of the space vehicle relative to the third reference axis which points towards the reference heavenly body, on the basis of the components of the total angular momentum vector of the space vehicle along the first two reference axes, and in controlling the reaction wheels to adjust the attitude of the space vehicle to take up the desired angle about the third reference axis.
To enable the space vehicle to be controlled about the third reference axis, the method consists in projecting the vector representative of the total angular momentum of the vehicle onto the transverse plane containing the first two reference axes, and in deducing the components of the vector along those two axes in order to compute the angle of said vector relative to one of the first two reference axes.
More precisely, in order to control the space vehicle about the third reference axis, the method consists in calculating a reference angle at a given instant corresponding to the desired angle of the space vehicle about the third reference axis, and then in calculating at each subsequent instant a xe2x80x9ccurrentxe2x80x9d angle of the space vehicle about the third reference axis to determine how the current angle is varying relative to the reference angle of rotation, and in bringing the value of said current angle to the value of the reference angle by acting on the speeds of rotation of the three reaction wheels.
According to the invention, the method consists in calculating the angle of rotation of the space vehicle about the third reference axis by applying the following formula:
xcex1=arctan 2(Hz/Hy)
where Hz and Hy are the respective components of the angular momentum vector of the space vehicle along the two first reference axes.
According to another characteristic of the method, the attitude of the space vehicle about the third reference axis is controlled after controlling its attitude about the first two reference axes.
Thus, according to an important advantage of the invention, the ability to measure the angle of rotation about the third reference axis enables the space vehicle""s attitude to be determined more accurately than can be done with the above-mentioned prior art method, where said angle of rotation is merely estimated, as opposed to being calculated.